In U.S. Pat. No. 3,663,084 issued May 16, 1972, there is disclosed a hollow retroreflector consisting of three plates joined together so as to form a hollow corner where three optically flat reflecting faces meet. In an example, a cluster of retroreflectors left on the moon is used to intercept light such as a laser beam from the earth and reflect the beam to earth. Retroreflectors have the distinctive property of reflecting the incoming beam back to its source without requiring any special or critical orientation of the retroreflector in relation to the beam. The efficiency or resolution of such a reflector depends importantly on the precision of the right-angled relationships of the reflecting faces and on their flatness.